Floods Chapter 11 Streams and Flood Processes Rising Waters Chapter 12 Floods and Human Interactions Floods Flooding of streams is one of the more common and costly types of natural disasters in the U S Account for one quarter to one third of annual disaster dollar losses Account for 80 of the annual disaster deaths The most common cause of flooding is PRECIPITATION Weather patterns determine precipitation Area over which the rain falls Duration of the rain Intensity The Hydrologic Cycle The Fate of Precipitation Water enters the atmosphere through evaporation Through precipitation water falls either directly into the ocean or onto the land Water that falls onto land enters streams by Infiltration The movement of water into rocks or soil through cracks and pore spaces Runoff Water that flows over the land How water gets to a stream The Drainage Basin A cup of land bounded by areas of high relief Precipitation that falls within the cup of land flows to the stream as runof over the surface or infiltration and Base flow through the groundwater system Example The United States is divided by the Appalachian Mountains on the East and the Rockies in the Midwest All water falling in between those two mountain ranges is in the Mississippi River Drainage basin Streams A body of running water that is confined in a channel and flows under the influence of gravity Channel width may vary from a few cm s to several km s Stream Gradient the vertical drop of a channel over a horizontal distance The headwaters are the upper part of the stream near its source in the mountains Upstream regions Steep Gradient Lower reaches of a stream are referred to as downstream regions Shallow Gradient The mouth is the place where a stream channel terminates and enters the sea a lake etc Base level is the theoretical limit to which the stream can erode It is in efect the elevation of the streams mouth In general streams begin at higher elevations and discharge into other streams and lakes relative base level that will eventually reach the ultimate base level sea level Potential Energy energy of position stored energy Kinetic Energy energy of motion energy to do work As potential energy is converted to kinetic energy the stream performs the work of erosion The amount of potential energy is proportional to stream gradient Erosion by streams has shaped the land surface worldwide over geologic time The ability of a stream to erode relates to stream Velocity the speed of the water generally measured in feet per second Discharge the total amount volume of water carried by the stream Discharge is generally measured in cubic feet per second or cfs Stream Gradation or Equilibrium The volume and velocity of stream flow limit both the size and the amount of sediment that can be carried by the stream A stream s COMPETENCE is the MAXIMUM grain size a stream can transport Faster moving water has a higher competence because it can move larger sized materials The VOLUME of sediment a stream can carry is called CAPACITY Larger streams having a larger volume of water have a higher capacity to transport stuf Streams will adjust their cross sections and channel gradient to accommodate the stream flow as well as the volume and grain size of sediment supplied to the channel Changes are insignificant with normal flow or even small floods and dramatic with large floods Upstream Narrow V shaped channels high velocity water cuts into stream bed High competence Downstream wide deep U shaped channel high volumes of water therefore wide and deep High capacity Discharge The volume of water passing a given point in a stream per unit time Q A v cross sectional area of the stream channel depth x width x velocity measured in m3 or ft3 per second Upstream Narrow V shaped channels High competence Downstream wide deep U shaped channel High capacity The Upstream Headwater region The collecting system high competence Steep gradient high velocity therefore high erosion Consist of a network of narrow v shaped tributary channels that collect water and sediment to the main stream The Downstream Region The transporting system high capacity Shallow gradient low velocity major process is to transport water and sediment from upstream to the streams mouth Consists of a wide deep U shaped channel called the main trunk stream Downstream channels tend to meander more and more over time The Downstream channel flows along a shallow gradient On either side of a downstream channel is a floodplain It is like a larger channel which encompasses the normal downstream channel As a stream rises prior to flooding its increased velocity and discharge allow it to carry more sediment When the water spills out onto the floodplain water velocity slows down and the sediment is deposited along the banks as levees The Dispersing System Consists of a network of distributaries at the streams mouth where water and sediment are dispersed Upon exiting the mouth the velocity of the stream will decrease to zero and the sediment is deposited to form a delta in water or an alluvial fan on land Flooding an excessive discharge When water falls within a stream s drainage basin it enters the stream by infiltration base flow and by runof contributing to the streams discharge The relationship between precipitation and the efect it has on a streams discharge is often shown in graphical form as a hydrograph The lag time is the time between peak precipitation and peak discharge flooding Flooding Infiltration vs Runoff High infiltration high residence time in the soil before entering the stream as base flow results in a long lag time and low peak discharge High runoff water that falls to the ground enters the stream quickly results in a short lag time and a high peak discharge Types of Floods Upstream Flash Floods Caused by locally intense rainfall covering only one or two tributaries over a short period of time Steep Gradient High and Rapid runof Narrow V shaped channels No floodplains or levees Waters rise quickly Short lag time and high peak discharge Q A X V Increased discharge increases water velocity resulting in an increase in erosion Unpredictable because it is a function of weather conditions high loss of life Flood damage mainly from the force of the rapidly moving water and deposition of sediment Floods rapidly recedes quickly Downstream unafected Covers a smaller area of land Flash Floods Rise rapidly and appear unexpectedly The force of the water can wash cars downstream and damage
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